It is known that public utility transport means, such as the vehicles used by the police and rescue services in general, are equipped with optical and acoustic signalling devices which are activated in the event of an emergency in order to signal the presence of the vehicles and request priority in heavy traffic conditions.
With the same aims, the same signalling devices may also be used on watercraft and airborne vehicles.
The combination of the acoustic signalling devices with the optical signalling devices is necessary because the former, generally a siren, by means of their sound indicate that there is an emergency, while the optical signalling devices allow precise determination of the direction from which the rescue vehicle is coming so that the traffic occupying the roadway may move aside and allow the vehicle to pass through.
The advantages of the signalling system are therefore based on the efficiency of both the signalling devices and in particular their use in combination with each other.
The optical and acoustic signalling devices of the known type present on the market and widely used have, however, certain drawbacks.
A first limitation consists in the fact that, while the acoustic signalling devices use a siren, the sound of which can be heard from far away in any environmental conditions, the visibility of the light beam emitted by the optical signalling devices is instead reduced, for example in the case of rain or fog.
In fact, the optical signalling devices of the known type use incandescent or halogen lamps which generate a light ray which is projected over a certain distance by a rotating directional dish, but which, because of the incoherent nature of the radiation forming it, is greatly absorbed by the water droplets which form the rain or the fog.
The result is that, in the event of particularly dense fog, the light ray is visible no more than a few metres from the lamp which emits it.
DE-A-4012120, which is considered the closest prior art to the invention, discloses an optical signaling device according to the preamble of claim 1.
Further optical signaling devices are known from US-B6183100, WOA-01/45980 and GB-A-2 360 350.
However, none of the optical signaling devices disclosed in such prior art documents permits to envelope the vehicle on which is mounted making it clearly visible at a great distance.